Cyclic esters are useful in a variety of applications. For example, cyclic esters derived from hydroxycarboxylic acids are useful in the preparation of environmentally biodegradable plastic materials, and for plastic materials which resorb when used in medical applications. Particularly useful are those polymers which are derived from .alpha.-hydroxycarboxylic acids, such as lactic acid, because they can be degraded over time by hydrolysis under most environmental conditions. The resulting hydroxy acid units (e.g., lactic acid) or oligomers thereof are then readily taken up by microorganisms in the environment and converted to carbon dioxide and water aerobically or carbon dioxide and methane anaerobically.
Cyclic esters, such as cyclic esters of hydroxycarboxylic acids can be produced through a number of mechanisms; however, such cyclic ester products typically contain multiple undesirable impurities. Such impurities can degrade cyclic esters in the product, resulting in short shelf life of the cyclic esters produced. For example, free acid and water in a cyclic ester composition can propagate hydrolyzing of the cyclic ester bonds, degrading the cyclic ester back into hydroxycarboxylic acids or other degradation products. In addition, impurities in the cyclic ester composition can interfere with both the rate of polymerization of the cyclic ester and the molecular weight, thereby prohibiting the formation of desirable higher molecular weight polymerization products. Accordingly, there is a need to obtain substantially pure cyclic esters, which are substantially free from impurities which degrade the compounds or interfere with subsequent chemical reactions, such as polymerization of the cyclic esters into high molecular weight polymers.
Several processes are known to purify synthesized cyclic ester compositions. Such processes include solvent crystallization, solvent scrubbing, solvent extraction, distillation, melt crystallization and sublimation. Although satisfactory results can be obtained in controlled laboratory conditions, many processes are difficult to operate on a commercial scale because they are complex or have impractical operating parameters. In addition, such processes may require high energy costs, high equipment costs or high reagent costs. Moreover, machinery may not be available for implementing these processes on a commercial scale.
Other processes are commercially unacceptable because the cyclic esters being purified may degrade due to the residence time or temperature constraints of the process, resulting in poor purity. Further, some processes provide low yields, making them economically unsuitable.
Accordingly, there is a need for inexpensive, reliable methods for purifying cyclic esters. In particular, there is a need for methods of obtaining cyclic esters of sufficient purity for use in producing high molecular weight polymers. Furthermore, there is a need for methods of purifying cyclic esters that allow for recovering all isomeric forms of the cyclic esters.